Epitope-tagging of a functional thyrotropin receptor: detection of the native receptor on intact cells

A comprehensive system for protein purification and biochemical analysis based on antibodies to c-myc peptide

The genomics revolution has created a need for increased speed and generality for recombinant protein production systems as well as general methods for conducting biochemical assays with the purified protein products. 9E10 is a well-known high-affinity antibody that has found use in a wide variety of biochemical assays. Here we present a standardized system for purifying proteins with a simple epitope tag based on c-myc peptide using an antibody affinity column. Antibodies with binding parameters suitable for protein purification have been generated and characterized. To purify these antibodies from serum-containing medium without carrying through contaminating immunoglobulin G, a peptide-based purification process was developed. A fluorescence polarization binding assay was developed to characterize the antigen-antibody interaction. Protein purification protocols were optimized using a fluorescein-labeled peptide as a surrogate "protein." Binding and elution parameters were evaluated and optimized and basic operating conditions were defined. Several examples using this procedure for the purification of recombinant proteins are presented demonstrating the generality of the system. In all cases tested, highly pure final products are obtained in good yields. The combination of the antibodies described here and 9E10 allow for almost any biochemical application to be utilized with a single simple peptide tag.

Anti-c-myc antibody 9E10: epitope key positions and variability characterized using peptide spot synthesis on cellulose

The 9E10 antibody epitope (EQKLISEEDL) derives from a protein sequence in the human proto-oncogen p62(c-myc) and is widely used as a protein fusion tag. This myc-tag is a powerful tool in protein localization, immunochemistry, ELISA or protein purification. Here, we characterize the myc-tag epitope by substitutional analysis and length variation using peptide spot synthesis on cellulose. The key amino acids of this interaction are the core residues LISE. The shortest peptide with a strong binding signal is KLISEEDL. Dissociation constants of selected peptide variants to the antibody 9E10 were determined. scFv constructs with the shortest possible myc-tags were successfully detected by Western blot and ELISA, giving a signal comparable to that of the original myc-tag.

Subunit structure of thyrotropin receptors expressed on the cell surface

We studied cell surface thyrotropin receptor (TSHR) by biotinylating proteins on the surface of metabolically labeled, intact cells. In addition to TSHR cleaved into A and B subunits, mature single-chain receptors with complex carbohydrate were also present on the cell surface. A low A/B subunit ratio indicated partial shedding of extracellular A subunits from transmembrane B subunits. TSHR cleavage at upstream site 1 (within amino acid residues 305-316) would generate a B subunit of 51-52 kDa. However, only smaller B subunits (40-46 kDa) were detected, corresponding to N termini from residues approximately 370 (site 2) extending downstream to the region of B subunit insertion into the plasma membrane. The intervening C peptide region between sites 1 and 2 could not be purified from TSHR epitope-tagged (c-myc) within this region. However, the small proportion of B subunits recovered with a c-myc antibody were larger (45-52 kDa) than the majority of B subunits recovered with a C-terminal antibody. In conclusion, our study provides the first characterization of cell surface TSHR including their A and B subunits. Single-chain, mature TSHR do exist on the cell surface. The C peptide lost during intramolecular cleavage disintegrates rapidly following cleavage at upstream site 1 of the single-chain TSHR into A and B subunits. N-terminal disintegration of the B subunit pauses at site 2, but then progresses downstream to the vicinity of the plasma membrane, revealing a novel mechanism for A subunit shedding.

Thyrotropin regulates c-Jun N-terminal kinase (JNK) activity through two distinct signal pathways in human thyroid cells

c-Jun N-terminal kinases (JNK) participate in cellular responses to mitogenic stimuli and environmental stresses. We investigated whether and how TSH, which promotes the proliferation and differentiation of thyroid cells, regulates JNK activity in primary cultured human thyroid cells. TSH stimulated JNK activity in cytosolic fractions of thyroid cells measured by in vitro kinase assay. A low concentration of TSH (10(-11) M) stimulated JNK activity but at a higher dose (10(-8)-10(-7) M), TSH suppressed JNK activity without any change of JNK protein level. Activation of JNK by TSH was also observed in CHO cells stably transfected with TSH receptor complementary DNA (cDNA), suggesting a ligand-receptor specific interaction. TSH stimulated JNK activity through a pertussis toxin-sensitive pathway. We next elucidated the signal transduction pathways in TSH-induced JNK activation by examining the involvement of four distinct intracellular signal molecules; protein kinase C (PKC), cAMP, Ca2+, and PI3-kinase. The stimulation of JNK by TSH was blocked by two PKC inhibitors and suppressed by 8-bromo-cAMP or forskolin. These findings demonstrate that TSH regulates JNK activity biphasically in human thyroid cells through an interaction between Gi-PKC and cAMP-PKA pathways.

Epitope tagging

Epitope tagging is a recombinant DNA method by which a protein encoded by a cloned gene is made immunoreactive to a known antibody. This review discusses the major advantages and limitations of epitope tagging and describes a number of recent applications. Major areas of application include monitoring protein expression, localizing proteins at the cellular and subcellular levels, and protein purification, as well as the analysis of protein topology, dynamics and interactions. Recently the method has also found use in transgenic and gene therapy studies and in the emerging fields of functional genomics and proteomics.

Role of asparagine-linked oligosaccharides in protein folding, membrane targeting, and thyrotropin and autoantibody binding of the human thyrotropin receptor

The amino-terminal ectodomain of thyrotropin (TSH) receptor (TSHR) is heavily glycosylated with asparagine-linked (N-linked) oligosaccharides. The present studies were designed to evaluate how acquisition and processing of N-linked oligosaccharides play a role in the functional maturation of human TSHR. A glycosylation inhibitor tunicamycin, which inhibits the first step of N-linked glycosylation (acquisition of N-linked oligosaccharides), and a series of mutant Chinese hamster ovary (CHO)-Lec cells defective in the different steps of glycosylation processing were used. Inhibition of acquisition of N-linked oligosaccharides by tunicamycin treatment in CHO cells stably expressing TSHR produced nonglycosylated TSHR, which was totally nonfunctional. In contrast, all of the TSHRs synthesized in mutant CHO-Lec1, 2, and 8 cells (mannose-rich, sialic acid-deficient, and galactose-deficient oligosaccharides, respectively) bound TSH and produced cAMP in response to TSH with an affinity and an EC50 similar to those in TSHR expressed in parental CHO cells (CHO-TSHR; sialylated oligosaccharides). However, Lec1-TSHR and Lec2-TSHR were not efficiently expressed on the cell surface, whereas the expression levels of Lec8-TSHR and CHO-TSHR were essentially identical. All of the TSHRs expressed in CHO-Lec cells cleaved into two subunits. Finally, anti-TSHR autoantibodies from Graves' patients interacted with all of the TSHRs harboring different oligosaccharides to a similar extent. These data demonstrate that acquisition and processing of N-linked oligosaccharides of TSHR appear to be essential for correct folding in the endoplasmic reticulum and for cell surface targeting in the Golgi apparatus. We also show that complex type carbohydrates are not crucially involved in the interaction of TSHR with TSH and anti-TSHR autoantibodies.

Sequence analysis and bacterial production of the anti-c-myc antibody 9E10: the V(H) domain has an extended CDR-H3 and exhibits unusual solubility

The cDNAs for the two variable domains of the antibody 9E10 were cloned from the hybridoma cell line. A chimeric 9E10 Fab fragment was produced in E. coli under control of the tightly controlled tetracycline promoter. The functional Fab fragment was isolated in a single step via a His6-tag, which also served for its recognition by a nickel chelate-alkaline phosphatase conjugate. Thus, the recombinant Fab fragment permitted the immunochemical detection of the myc tag in a sandwich ELISA. The dissociation constant for the interaction with the myc tag peptide was determined as 80 +/- 5 nM by fluorescence titration. In an attempt to produce the smaller 9E10 Fv fragment it was found that its V(H) domain alone can be readily isolated from E. coli as a soluble protein. This unusual behaviour may be explained by the 18 amino acid-long CDR-H3 and could be of value in the design of 'single domain' antibodies.

Evidence that the thyrotropin receptor ectodomain contains not one, but two, cleavage sites

TSH receptor (TSHR) cleavage into two subunits (A and B) was explored using two new mammalian cell lines expressing the recombinant receptor; 1) TSHR-10,000 CHO cells overexpressing the TSHR; 2) TSHRmyc cells with a c-myc epitope inserted at residues 338-349. Immunoprecipitation or immunoblotting of TSHR-10,000 cells with mAb to either the A subunit or the B subunit revealed multiple forms of the TSHR: 1) uncleaved receptors of approximately 115 kDa and approximately 100 kDa with complex carbohydrate and high mannose carbohydrate, respectively; 2) two subunit TSHR with an approximately 62 kDa A subunit containing complex carbohydrate. The A subunit was approximately 35 kDa after enzymatic deglycosylation (predicted C-terminus near residue 330). The nonglycosylated B subunit was evident primarily as an approximately 42 kDa band (predicted N terminus near residue 380). The sum of the A and B subunit polypeptide backbones was smaller than the predicted size of the TSHR, a polypeptide backbone (84.5 kDa), raising the possibility that an approximately 5-kDa polypeptide fragment was excised during intramolecular cleavage. This hypothesis was supported by data obtained with the TSHRmyc cells. Thus, mAb to the c-myc epitope and to amino acid residues 22-35 (mAb A10) were equally effective in detecting the single chain forms of the TSHR in these cells. However, the 35 kDa, deglycosylated A subunit was clearly visible on immunoprecipitation with mAb A10 to the TSHR amino terminus, but not with the anti-myc mAb, indicating loss of the c-myc epitope at residues 338-349. Further, even though the A subunit was not detected in TSHRmyc cells with anti-myc mAb, 125I-TSH cross-linking to the cell surface showed similar A subunit expression in TSHRmyc and wild-type TSHR expressing cells. In summary, our study provides a surprising and novel finding for G protein-coupled receptors. Contrary to the prevailing concept of one cleavage site in the TSHR, we present evidence that there are, in fact, two such sites. The TSHR, like insulin, may release a C peptide during intramolecular cleavage into two subunits.